In the cold rolling of metals it is normally necessary to supply copious quantities of a liquid to provide lubrication and cooling of rolls and the rolled strip. The energy of deformation of the rolled product appears as heat, which must be removed by the coolant in order to avoid excessive temperature of the strip and of the rolls. When rates of deformation are high, the rate of heat generation is high, and high coolant flow rates are necessary. For example, a Sendzimir mill rolling 50 inch wide stainless steel typically uses a coolant flow rate of around 1000 gallons per minute.
In general, the coolant medium is oil based, and typical types used are straight mineral oil, water/oil emulsions, (usually containing about 5 to 12% oil) either based upon mineral oils, or upon vegetable oils such as palm oil. It is usual to incorporate chemical additives in the oil, such as extreme pressure (E.P.) additives to increase film strength, anti-oxidants, anti-rust compounds (in case of water/oil emulsions), anti-foam compounds and so on.
Subsequent to the rolling process the strip is frequently transferred to a process for which it is necessary to have little or no coolant (or additives) remaining on its surface. For example, when coils are batch annealed, surface oil can cause "stickers" (where adjacent laps of the coil stick together) and "staining" (discoloration of strip surface due to impurities or additives in the coolant). In some cases the strip can be cleaned before being transferred to the next process, but even then, cleaning is an additional cost to the strip processing operation, and elimination or reduction of the requirement for cleaning is a very worthwhile objective. Furthermore, even coiling of strip with an oily surface is hazardous, particularly at light gauges, because "telescoping" of the coil can readily occur ("telescoping" is when adjacent laps of the coil slide over one another in an axial direction, causing the strip emerging from the rolling mill to be forced to the side, thus producing a mill wreck). Surface oil can also adversely affect the measurement accuracy of strip thickness measuring equipment.
For this reason, it is usual to provide cold rolling mills with strip wipers, which have the function of wiping the oil from the surface of the strip as it emerges from the mill, usually before the strip reaches the strip thickness gauges.
There are three basic types of strip wipers:
(1) Bar wipers
These consist of two strips or bars of a suitable wiping material, one above and one below the strip. These bars are mounted transverse to the strip, and extend beyond the strip edges. They are provided with rigid support frames and are squeezed together against the strip under pressure. Thus they provide a "squeegee" like action. Typical materials for the bars are rubber hose (commonly used for mineral oils) and felt (commonly used for soluble oils).
Bar wipers have three major defects. Firstly, the bars are damaged very quickly by the strip edges, so have to be changed frequently (delays production). Secondly, any metal chips produced by the rolling process tend to become embedded in the material of the bars, and will then scratch the strip surface. Thirdly, they provide a high friction drag on the strip, which can result in strip breakage, particularly at light gauges, and if the strip edges are not smooth.
(2) Air flow type wipers
These wipers required a continuous flow of air generated either by a compressor or fan (air jet type) or by a vacuum ejector or fan (vacuum type).
The action of these wipers is that of sweeping the coolant off the surface of the strip by high velocity air flow. This is the same principle as that of the vacuum cleaner.
Air jet type wipers are inexpensive to install, but expensive to run due to the cost of supplying compressed air. They are not very effective, and are usually noisy.
Vacuum type wipers, usually operating in combination with bar wipers are quite effective, especially when rolling very light gauge materials at high speed. However, they are expensive to buy, and expensive to run due to the cost of running vacuum ejectors or fans.
(3) Roller wipers
There are two types of roller wipers - two roll wipers and three roll wipers.
Two roll wipers are similar to bar wipers except that the wiper bars and support frames are replaced with two rollers, usually made of steel or bronze, one mounted above the strip, vertically above a second one mounted below the strip. These rollers are mounted in chocks at their ends, and the force which squeezes them together against the strip is applied to the chocks. Thus the rollers tend to bend under load, and must usually be provided with a crown, in order to apply uniform pressure and thus uniform wiping action at all points across the strip. Two roll wipers have the advantage that damage due to strip edges is minimal, and they will usually not scratch the strip, but have the disadvantage firstly that their wiping, at its best, is inferior to that of bar wipers (probably because the oil film can penetrate the nip area due to hydrodynamic lift effects) and also that, if the strip profile is irregular, or if the roll crowns are not exactly suitable for the applied wiping pressure, then wiping will be non-uniform, and bands of oil will be visible on the strip as it emerges from the wipers.
Three roll wipers utilize two rollers, side by side (i.e. with axes parallel) above the strip, and a single roller below the strip, with its axis vertically below a line mid-way between the axes of the upper rollers. Note that it is possible to have two rollers below, and one above the strip, but it is better practice to use two rollers above the strip and one below, since more coolant has to be removed from the upper side of the strip. For three roll wipers to be effective the strip must be under tension (which is the usual condition when cold rolling) since although the upper and lower rolls are urged towards each other by hydraulic cylinders (or other means) in order to press them against upper and lower strip surfaces, the pressure with which they are urged must not be strong enough to cause the rollers to "pinch" the strip, but must be strong enough to give good wiping action. Correct operation of 3-roll wipers is illustrated in FIG. 3 where it can be seen that between each roll/strip contact zone and the next there is a free length of strip 110 (under tension). This free length of strip is very important, since it is able to flex, so that, even if the crowns in upper and lower wiper rolls do not exactly match their deflections, substantially uniform wiping pressure will be obtained in each roll/strip contact zone, across the full width of the strip. Since it is only the strip tension, acting via the wrap angle of the strip around each roll which balances the pressure applied by the hydraulic cylinders, it follows that the strip tension must be sufficiently high to balance the applied pressure.
Prior art three roll wipers wipe more uniformly than two roll wipers and offer all the other advantages of two roll wipers. However, they do not wipe as effectively as bar wipers so cannot be used in critical applications.
Typical construction of prior art 3-roll wipers is shown in FIGS. 1 and 2. Wiper rollers 11 are hollow, and are rotatably mounted upon "dead" or stationary shafts 13 by means of ball bearings 12. Shafts 13 are mounted in chocks 14 (upper roller) and 15 (lower roller), spaced from roller ends by spacers 26. Lower chocks 15, are slideably mounted in frames 16, which are attached to mill housings 20 and 21 by means of screws 24'. Hydraulic cylinders 18 mounted to mill housings 20 and 21 by means of brackets 19 and screws 22 are used to set the upper rollers in a fixed horizontal position so that their lower generators lie on the pass line, by preloading upper chocks 14 against frames 16 via spacers 17. Hydraulic cylinders 23, mounted upon frames 16, are supplied with hydraulic oil at an adjustable constant pressure, and are used to urge the lower wiper roller upwards against the lower surface of strip 25.
Our experience with prior art three roll wipers on a number of installations has taught us that smaller wiper rollers and harder roller materials remove oil from the strip more effectively. This can be explained by the fact that with harder materials and smaller roller diameters, the contact area between wiper roller and strip (for a given applied force) is smaller, and hence the average pressure between roller and strip is higher, enabling each roller to "bite" through the oil film more effectively. However, using the prior art method of mounting wiper rollers in chocks, it is not practical to reduce the roller diameter to less than about 1/12 of the strip width. Furthermore, at diameters of this size, the space taken up by the three roll wipers in the direction of strip travel, which usually has to be about 21/2 roller diameters as can be seen from FIG. 2, is relatively large, and it is thus frequently impossible to retrofit such wipers on to existing rolling mill installations, because there is insufficient space. It is also found that, at least on the larger (50 or 60 inch wide) installations, the wiper rollers are heavy, and changing of rollers can be very time consuming.